Method of induction heating and quenching

ABSTRACT

An induction hardening heat treatment method comprises the steps of induction heating a metal part or component, particularly a part fabricated of a high strength material, and then exposing the part to a sequence or series of partial, intermittent or interrupted quenches. Such intermittent or interrupted quenching achieves the necessary change in surface temperature with time to achieve a martensitic transformation of the surface adjacent metal or material while minimizing surface to core temperature differentials during the process which could result in cracking of the part, especially if it is fabricated of high strength materials.

FIELD

The present disclosure relates to a method of induction hardening of metals and more particularly to a method of induction heating and intermittent quenching of metals, particularly high strength materials, which reduces or eliminates cracking.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.

Post fabrication treatment of fabricated metal parts such as gears, shafts, sprockets, bearings and similar components is commonplace. The usual reason for such treatment is a desire or need to increase the strength and durability of the part and most processes involve heating the part, followed by controlled cooling. Due to the available excellent process control, induction hardening, which involves induction heating of the part followed by a controlled quench, is a preferred method of strength increase for many automotive parts such as gears, sprockets and shafts. Induction hardening provides a surface adjacent region or case of increased hardness which may be in the range of from 37 to 58 HRC, thus the frequently used term “case-hardened.”

The strength, load and service requirements of a particular component may be such that even with post fabrication treatment, it must initially be fabricated of a high strength material whose strength is further increased by treatments such as the induction hardening process. The use of such high strength materials and the sophisticated and complex geometry of some parts presents an additional challenge, namely, the possibility or likelihood of cracking during the induction hardening quench. Such cracking is a direct result of the rapid temperature reduction of the part which is necessary to achieve the desired or necessary hardness and more particularly the temperature differential between the surface and the core of the part as it is quenched which generates internal stresses. Unfortunately, the rapid temperature reduction is the source or mechanism of strength increase, the cracking being but a highly undesirable side effect of such process.

The present invention is directed to a method of post fabrication strength increase including induction hardening (heating and quenching) which minimizes or eliminates cracking of the treated part, especially parts fabricated of high strength materials.

SUMMARY

The present invention provides an induction hardening heat treatment method comprising the steps of induction heating a metal part or component to at least its austenitic temperature, particularly a part fabricated of a high strength material, and then exposing the part to a sequence or series of partial, intermittent or interrupted quenches. Such intermittent or interrupted quenching achieves the necessary change in surface temperature with time to achieve a martensitic transformation while minimizing surface to core temperature differentials which could result in cracking of the part, especially if it is fabricated of high strength materials.

Thus it is an aspect of the present invention to provide a method of induction hardening fabricated metal parts.

It is a further aspect of the present invention to provide a method of induction hardening parts fabricated of high strength materials.

It is a still further aspect of the present invention to provide a method of heat treating a fabricated metal part including the step of induction heating the part.

It is a still further aspect of the present invention to provide a method of heat treating a fabricated metal part including the steps of induction heating the part and intermittently and repeatedly quenching the part.

It is a still further aspect of the present invention to provide a method of heat treating a part fabricated of a high strength material including the steps of induction heating the part and exposing the part to a series of interrupted, partial quenches.

It is a still further aspect of the present invention to provide a method of induction hardening a fabricated metal part including the steps of induction heating the part and intermittently and repeatedly quenching the part.

It is a still further aspect of the present invention to provide a method of induction hardening a part fabricated of a high strength material including the steps of induction heating the part and exposing the part to a series of interrupted, partial quenches.

Further aspects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIGS. 1A, 1B and 1C are schematic diagrams of a sequence of induction hardening steps according to the present invention which occur at a work station including an induction heating step, a partial spray quenching step and a dwell step, respectively;

FIG. 2 is a flow diagram for an induction hardening process according to the present invention; and

FIG. 3 is a time-temperature-transformation diagram of a typical steel alloy presenting temperature on the vertical (Y) axis, time on the horizontal (X) axis and data from a typical and representative intermittent quench operation according to the present invention.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

With reference to FIG. 1A, a work station for heat treating by induction hardening of parts or components such as gears, sprockets, shafts, bearings and the like is illustrated and generally designated by the reference number 10. The heat treatment work station 10 includes a turntable or fixture 14 which is rotated at a desired speed by a motor and gear drive assembly 16. It will be appreciated that the turntable or fixture 14 may take various forms and configurations which adapt it to and facilitate mounting or securement of variously shaped and configured parts or components 20 such as gears, sprockets, shafts, bearings or similar metal components thereto. Additional transport devices or mechanisms (not illustrated) may and typically will be utilized to load and unload the turntable or fixture 14.

The turntable or fixture 14 and the part or component 20 secured thereto are disposed and rotated within the heat treatment work station 10 which includes an induction heater 24 having one or more electromagnetic coils 28 which surround the part or component 20. The electromagnetic coils 28 are supplied with alternating current at a power level and frequency that effectively and efficiently heats the part or component 20. While different temperatures and temperature ranges will be appropriate for different materials and alloys as well as different sizes and configurations of parts or components 20, a gear or sprocket having a diameter between about 80 and 150 millimeters (3.15 to 5.91 inches) and a thickness between about 10 and 20 millimeters (0.394 to 0.787 inches) with external teeth which is fabricated of a material such as D700 nodular iron per SAE standard J434 is preferably heated to a temperature in the range of 840° C. (1545° F.) to 950° C. (1740° F.). Given the variability of materials, parts and components 20, shapes and sizes, it should be understood that specific temperatures both within and without this recited temperature range are within the purview of the present invention.

Referring now to FIG. 1B, after heating of the part or component 20 to the desired temperature in the induction heater 24, electrical power to the electromagnetic coils 28 is terminated. The heat treatment work station 10 also includes a plurality or array of spray heads or nozzles 32 which are selectively supplied with a quenching solution from a controlled and pressurized source (not illustrated) of such solution which is primarily water and 3% to 9% polymer additive. The polymer additive may be one of many suitable known and available polymer quench additives. While the turntable or fixture 14 continues to rotate, a first, partial quench occurs as the part or component 20 is subjected to quenching by a spray of the quenching solution from the spray heads or nozzles 32 for one to five seconds. Continued rotation of the turntable or fixture 14 is desirable as it improves the uniformity of the partial quench. Given larger and heavier parts or components 20 or those fabricated of other materials, the first quench time may be longer and for smaller and lighter parts or components 20, the first quench time may be shorter.

Referring now to FIG. 1C, the first, partial quench is interrupted by stopping the flow of the quenching solution to the spray heads or nozzles 32. The quenching process then dwells for a first five to ten second interval. During this time, the turntable or fixture 14 and the part or component 20 may cease to rotate or they may continue to rotate. Once again, the recited dwell time interval may, in a given situation, be longer or shorter, depending upon the variables recited above.

Referring again to FIG. 1B, after the first dwell or interval, a second, partial quench is begun by restarting the flow of quenching solution to the spray heads or nozzles 32 and, if stopped, the turntable or fixture 14 and the part or component 20 are again rotated for a second ten to twenty second interval.

Referring again to FIG. 1C, after this second, partial quench, the supply of quenching solution to the spray heads or nozzles 32 is again terminated and a second interruption or dwell interval of ten to twenty seconds then occurs. Again, during this time, the turntable or fixture 14 and the part or component 20 may continue to rotate or their rotation may be stopped.

Returning again to FIG. 1B, the part or component 20 is next subjected to a third, partial quenching interval of ten to thirty seconds. This third, partial quench is achieved by again starting the flow of quenching solution through the spray heads or nozzles 32 and rotating the turntable or fixture 14 and the part or component 20 if it was stopped for the second dwell interval. For many parts and components 20, the third, partial quench is sufficient to complete the quenching process. If the part or component 20 is then fully heat treated (induction hardened) as described more fully below, it may be released or removed from the turntable or fixture 14.

Once again, it should be understood that the quench and interrupt times recited above are nominal and effective values for the gear or sprocket described in Paragraph [0021], above, and that other time intervals and sequences including more or fewer and longer or shorter duration quenches and more or fewer and longer or shorter interruptions may, and typically will, be appropriate for other parts and components 20. Furthermore, although FIGS. 1A, 1B and 1C illustrate a heat treatment work station 10 having both an induction heater 24 and a plurality or array of spray heads or nozzles 32, the interrupted or sequential, partial quenching process of the present invention may also be accomplished in (1) a heat treating line in which a first station includes an induction heater and a second, separate station includes one or a plurality of spray nozzles or heads or (2) a heat treating line which includes an induction heater and one or a plurality of baths or tanks (not illustrated) in which the partial quenching steps occur. It should be understood that these and other quenching facilities are capable of providing the repeated, multiple partial quenches of the present invention.

Turning now to FIGS. 1 and 2, a process flow chart of the induction hardening steps according to the present invention is designated by the reference number 50. The process flow chart 50 includes an initial step 52 of fabricating a part or component 20. The part or component 20 may by a conventional metal or metal alloy or it may be fabricated of a high strength material. As noted above, such high strength materials are often prone to cracking during induction hardening and thus the present invention is especially appropriate and beneficial when utilized with such materials.

Next, the part or component 20 is heated in an induction heater 24 in a process step 54. The part or component 20 is then subjected to a first partial quench in a process step 56. The first partial quench of the process step 56 extends over approximately 5% to 10% of the nominal total quench time for the part or component 20. Next is a first dwell step 58 that extends over approximately 30% to 35% of the total dwell time. A second partial quench occurs in the process step 62 which occupies approximately 30% to 50% of the nominal total quench time for the part or component 20. A second dwell step 64 follows which extends over approximately 65% to 70% of the nominal total dwell time. A third partial quench step 66 occupies approximately 50% of the nominal total quench time. Assuming appropriate initial temperature and quench and dwell times for the nature of the part or component 20, it will likely be heat treated and induction hardened at this time.

Two final steps may be undertaken which are not, per se, inherent or necessary process steps but are primarily diagnostic, that is, optional steps that may be undertaken or performed to determine whether the process has produced a properly heat treated part or component 20. Accordingly, a decision point 70 is entered which inquires if bainite has been created in the part or component due to the slowness of the quench. If it has, the decision point is exited at YES and a process step 72 is entered which designates the part or component 20 as unsatisfactory or out of specification. A process step 74 is then entered which adjusts the various quench cycles to increase the quench rate such that bainite is not formed. If no bainite is formed, the decision point 70 is exited at NO and a second decision point 76 is encountered which inquires whether a surface martensitic transformation has occurred in the part or component 20. If it has not, the decision point 76 is exited at NO and the process flow 50 returns to the induction heating step 54 to repeat. If it has, the decision point 76 is exited at YES and the process concludes at the endpoint 78.

Referring now to FIG. 3, a time-temperature-transformation diagram generally illustrating the various phases of a typical steel alloy and the process of the present invention appear together. The vertical (Y) axis represents temperature and the horizontal (X) axis represents time with the values of both increasing from their point of intersection. On the right of the diagram, various phase regions such as austenite, pearlite, bainite and martensite are presented. To the left of the diagram, represented by the line 80 is the induction heating step 54. The line 80 represents both the surface temperature and the core temperature of the part or component 20 and is therefore somewhat wide to indicate that the two temperatures may not, and typically will not, be the same. Although it is desirable that the surface and core temperatures of the part or component 20 be the same, or very nearly so, at the conclusion of the heating step 54/beginning of the first quench represented by the point 82, the austenitic temperature, this can typically be achieved only with significant engineering effort and development which is generally viewed as of marginal value and benefit.

The stepped line 84 which descends from the point 82 represents the surface temperature of the part or component 20 during the intermittent, that is, quench and dwell, steps 56, 58, 62, 64 and 66 set forth above. The slightly irregular line 86 which is adjacent the line 84 represents the core temperature of the part or component 20. Note, first of all, that the line 84 terminates in the martensitic region 88 indicating a martensitic transformation of a surface adjacent region of the part or component 20 and that the bainite region 92 has been avoided. Second of all, there is never a significant differential between the temperatures of the surface, line 84, and the core, line 86. This relative conformity between the surface and core temperatures of the part or component 20 during the quenching stages of the induction hardening process minimizes the generation of internal stresses and thus minimizes or eliminates cracking of parts or components 20, especially those fabricated of high strength metal alloys and materials. For purposes of comparison, the line 94 represents the surface temperature of a part or component 20 undergoing a conventional, rapid and continuous quench. Note that during the latter portion of the quench, a significant temperature differential exists between the core temperature, the line 86, and the surface temperature, the line 94.

It will be appreciated that the foregoing description has enabled and described the best mode contemplated by the inventors for practicing the invention. As noted above, because of the wide variations of possible metals, materials and sizes and configurations of parts and components 20 that will benefit from the heat treating process described herein, it should be understood that more or fewer partial quenching steps, interrupted by more or fewer sequential rest or dwell periods as well as different times for both the quench and dwell periods may, and likely will, be appropriate for other parts and components 20.

The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention. 

1. A method of induction hardening a part or component, comprising the steps of: induction heating the part or component, subjecting the part or component to a first, partial quench, interrupting said first, partial quench for a first interval, subjecting the part or component to a second, partial quench, interrupting said second partial quench for a second interval, and subjecting the part or component to a third, partial quench.
 2. The method of claim 1 wherein said first, partial quench is shorter in duration than either said second, partial quench or said third, partial quench.
 3. The method of claim 1 wherein a surface adjacent region of said part or component undergoes a martensitic transformation during said quenches.
 4. The method of claim 1 further including the step of determining that a surface adjacent region of said part or component has undergone a martensitic transformation during said quenches.
 5. The method of claim 1 wherein the part or component is induction heated to at least the austenitic temperature.
 6. The method of claim 1 wherein said second, partial quench and said second interval are substantially equal in duration.
 7. The method of claim 1 wherein said duration of said second, partial quench is longer than said first, partial quench and said third, partial quench is longer than said second, partial quench.
 8. A method of induction hardening a metal part, comprising the steps of: induction heating said metal part to at least its austenitic temperature, partially quenching said metal part, interrupting said partial quenching for a time interval, and repeating said partial quenching and interrupting steps until a surface adjacent region of said metal part has undergone a martensitic transformation.
 9. The method of induction hardening a metal part of claim 8 wherein a total of three partial quenching steps are performed.
 10. The method of induction hardening a metal part of claim 8 wherein a first partial quenching step is shorter in duration than a subsequent partial quenching step.
 11. The method of induction hardening a metal part of claim 8 wherein a second partial quenching step and a second partial quenching interruption are substantially equal in duration.
 12. The method of induction hardening a metal part of claim 8 wherein said duration of said partial quenching steps is increased if bainite is detected in said metal part.
 13. The method of induction hardening a metal part of claim 8 wherein a small differential between a surface temperature and a core temperature of said metal part are maintained during said repeated quenching and interrupting steps whereby stress cracking is essentially eliminated.
 14. A method of induction hardening a metal part comprising the steps of: providing an induction heater and induction heating a metal part, providing a quenching station and partially quenching said metal part at said quenching station in a first quenching partial step, interrupting said first partial quenching step and holding said metal part for a first dwell period in said quenching station, further partially quenching said metal part at said quenching station in a second, partial quenching step, interrupting said second partial quenching step and holding said metal part for a second dwell period in said quenching station, and further quenching said metal part at said quenching station.
 15. The method of induction hardening a metal part of claim 14 wherein said first partial quenching step is shorter in duration than a subsequent partial quenching step.
 16. The method of induction hardening a metal part of claim 14 wherein said second partial quenching step and said second dwell period are substantially equal in duration.
 17. The method of induction hardening a metal part of claim 14 wherein said duration of said partial quenching steps is increased if bainite is detected in said metal part.
 18. The method of induction hardening a metal part of claim 14 wherein said metal part is heated to at least its austenitic temperature.
 19. The method of induction hardening a metal part of claim 14 wherein a small differential between a surface temperature and a core temperature of said metal part are maintained during said repeated quenching and interrupting steps whereby stress cracking is essentially eliminated. 